Conductivity in semiconductor devices can be achieved by various means. The conductive lines on different levels are usually connected through conductive plugs at a specific position. However, the number of building block, for example, a transistor, of the vast majority of semiconductor devices increases greatly. In a single semiconductor device, such as a high performance processor device, can includes millions of transistors. Therefore, decreasing transistor size and increasing its density is a great concern in the field since the device dimension inhibits any physical expansion of the transistor.
A conventional semiconductor device may include forming a contact hole in an inter layer dielectric (ILD) layer and then filling the contact hole with a conductive material. As the size and spacing of the transistors decreases, the aspect ratio of the contact hole increases significantly. As a result, it is more difficult to accurately and repeatedly form stacked contact structure.